1. Field of the Invention
The present invention relates to a hologram recording medium for providing improved security of holograms used for authentication purposes.
2. Description of the Related Art
Three-dimensionally displayable holograms are used to ensure that CD cards, prepaid cards, passes, bank account books, passports, identification papers, merchandise and others are legitimate and not falsified. At present, it is widely common to affix, for example, a label or sticker to the target. The label or sticker includes an embossed hologram. In the embossed hologram, interference fringes are recorded as projections and depressions on the surface. However, embossed holograms are easy to counterfeit. In contrast, Lippmann holograms, in which interference fringes are recorded as a difference in refractive index within the film, are extremely difficult to counterfeit. One of the reasons for this is that advanced techniques are desired to produce a recorded image. Another reason is that recording media are difficult to procure.
There are two production methods for a volume hologram (also referred to as a Lippmann hologram), namely, real image hologram and holographic stereogram. To prepare a real image hologram, a laser is irradiated onto the subject. In contrast, a holographic stereogram is recorded based on parallax images from a number of viewpoints. The production process of a volume holographic stereogram includes three steps, i.e., a content production step which includes image acquisition and editing and other processing of the acquired images, a hologram master preparation step and a copying (mass production) step. The images are acquired by image capture or computer graphics. Each of a plurality of images acquired in the image editing step is converted to a strip-shaped image, for example, by a cylindrical lens. The master is prepared by sequential recording of interference fringes between the image object beam and the reference beam as strip-shaped elementary holograms to the hologram recording medium. The hologram is reproduced by contact printing using the master. That is, the hologram recording medium is brought into intimate contact with the master, followed by irradiation of a laser beam for reproduction of the hologram. As a result, the hologram is copied.
In this hologram, image information acquired by sequential shooting from different viewing points in the horizontal direction is sequentially recorded in the horizontal direction as strip-shaped elementary holograms. When the viewer looks at this hologram with both of his or her eyes, the two-dimensional images seen by the left and right eyes are slightly different. As a result, the viewer perceives parallax, thus reproducing the three-dimensional image.
As described above, in order to sequentially record strip-shaped elementary holograms, an HPO (Horizontal Parallax Only) holographic stereogram is produced which has parallax only in the horizontal direction. An HPO holographic stereogram desires only a short printing time and yet provides high image quality in recording. Further, vertical parallax can be created depending on the recording format. A hologram having parallax in the horizontal and vertical directions is referred to as an FP (Full Parallax) hologram.
A volume hologram sticker has been proposed (refer to Japanese Patent Laid-Open No. 2002-82593 (hereinafter referred to as Patent Document 1)). This sticker is designed to be affixed to an identification paper and may not be reused if peeled off. The reason for this is that the hologram layer is damaged because the peeling strength of the hologram layer is greater than the breaking strength thereof. In Japanese Patent No. 3342056, on the other hand, a hologram has been proposed in which a polarization control layer is provided. However, the polarization control layer itself is not patterned. Therefore, this hologram has been effective in aggravating the overall efficiency for copying the image, but failed to be significantly effective to such an extent that the latent image becomes apparent.